1. Field of the Invention
The present invention relates to a bank quantity adjusting device for an extruder for extruding a polymeric material such as rubber to form a sheet.
2. Description of the Prior Art
This kind of extruder is provided with a roller die consisting of a pair of upper and lower rollers on the downstream side of an outlet of an extruder body. A material supplied from a batch type mixer into the extruder body is extruded by a screw provided in the extruder body, and is then allowed to pass between the pair of upper and lower rollers of the roller die, thus forming a sheet from the extruded material. In such an extruder, a bank quantity of the material accumulated in a bank portion defined between the outlet of the extruder body and the roller die must be properly controlled to uniform the shape (width, thickness, etc.) of the sheet to be discharged.
A control method for the bank quantity is described in Japanese Patent Publication No. 55-4575, for example. According to this method, the bank quantity is controlled by detecting a bank pressure and then controlling a screw speed or a roller die speed on the basis of the bank pressure detected. In this conventional method including detection of the bank pressure, however, the bank quantity is not proportional to the bank pressure because of a variation in hardness of rubber due to a difference in composition of the material. Accordingly, accurate control is difficult to perform.
Another control method for the bank quantity is described in Japanese Patent Laid-open No. 56-118841, for example. According to this method, the bank quantity is controlled by detecting a bank quantity and then controlling a screw speed or a roller die speed on the basis of the bank quantity detected. In this conventional method including detection of the bank quantity, however, there is a problem that a movable portion for detecting the bank quantity is clogged with rubber. Examples of such a bank quantity detecting device for detecting the bank quantity are described in Japanese Patent Laid-open Nos. 4-10918 and 5-104607. The former discloses a contact type of bank quantity detecting device, while the latter discloses a noncontact type of bank quantity detecting device.